In an automobile engine, a cold-water cooling system employing a radiator is generally used for cooling such [engine]. From the past, in order to control the temperature of the coolant to be introduced into the engine in this type of cooling system, a thermostat employing a thermal expansion body for adjusting the amount of coolant to be circulated on the radiator side, or a valve unit for electrically controlling the same, is being used.
In other words, the thermostat employing the thermal expansion body or the control valve formed from the likes of an electrical control valve unit described above is installed to a part of the coolant passage; for instance, to the inlet side or the outlet side of the engine, and, when the coolant temperature is low, the control valve is closed to circulate the coolant via a bypass channel without passing through the radiator, and, when the coolant temperature becomes high, the control valve is opened to circulate the coolant through the radiator so as to control the coolant temperature to be a required state.
The overall schematic of an automobile engine cooling system (coolant temperature control system) employing a thermostat is now explained with reference to FIG. 6.
In FIG. 6, reference numeral 1 represents an automobile engine as the internal combustion engine constituted with a cylinder block and cylinder head, and a coolant passage represented with arrow a is formed inside the cylinder block and cylinder head of this engine 1.
Reference numeral 2 represents a cooling unit, or a radiator, and a coolant passage is formed in this radiator 2 as universally known. A coolant inlet 2a and a coolant outlet 2b of the radiator 2 are connected with a coolant circuit 3 for circulating the coolant in the engine 1.
This coolant circuit 3 is constituted of an outflow side cooling channel 3a communicating from the coolant outlet 1c provided to the engine 1 to the coolant inlet 2a provided to the radiator 2, an inflow side cooling channel 3b communicating from the coolant outlet 2b provided to the radiator 2 to the coolant inlet 1b provided to the engine 1, and a bypass channel 3c for connecting the portions in the middle of such cooling channels 3a, 3b. 
The engine 1, radiator 2 and cooling channel 3 form the coolant circuit.
A thermostat device 5 for controlling the flow and amount of coolant in this kind of coolant circuit 3 according to the coolant temperature is provided [at a position] in the middle of the cooling channel 3b on the inlet side of the engine 1, and at an intersection enabling the switching control of the coolant from the radiator 2 and the bypass passage 3c. Incidentally, reference numeral 3d in the diagram represents a cooling channel leading from such intersection to the inlet 1b of the engine 1.
Further, although not shown in FIG. 6, disposed to the inlet 1b portion of the engine 1 is a water pump for coercively circulating the coolant inside the cooling channel 3 as a result of the rotational axis being rotated pursuant to the rotation of a crankshaft (not shown) of the engine 1. Further, reference numeral 6 in the diagram represents a cooling fan unit for coercively taking cool air into the radiator 2, and is constituted of a fan and an electric motor for rotationally driving such fan.
Flow of the current in this kind of coolant circuit 3 is switch-controlled with the thermostat device 5. In other words, the constitution is such that, when the coolant temperature is low, the coolant is circulated via the bypass channel 3c, and when the coolant temperature becomes high, the coolant is circulated on the radiator 2 side, and not via the bypass channel 3c, and then supplied to the engine 1.
With the thermostat device 5, as shown in FIG. 7, a first valve disc 8 is provided to one end (upper end in the drawing) of the operating member 7 which operates pursuant to the temperature change of the fluid, a second valve disc 9 is provided to the other end (lower end in the drawing) of such operating member 7, and comprises a coil spring 10, which is a biasing means, for biasing the first valve disc 8 to the valve-closing position, and a main frame 11.
The operating member 7 is a so-called thermoelement, is formed from a temperature sensor 7a and a guide unit 7b, a thermal expansion body 7c such as wax that expands and contracts upon sensing the temperature of the fluid is built in the temperature sensor 7a, and a piston rod 7d is fitted into the guide unit 7b extending from the tip of the temperature sensor 7a. Further, provided to the tip of the piston rod 7d is a pressing holder 12 for pressing down on the tip of the piston rod 7d. 
The first valve disc 8 is provided to the guide unit 7b, and the pressing holder 12 is made to be the valve seat of the first valve disc 8. Further, a mounting unit 12 with a water channel is provided protrusively to the outside of the pressing holder 12. Reference numeral 12 represents packing.
The second valve disc 9 is mounted with a stopper 13a to a valve rod 13 extending from the back end of the temperature sensor 7a, and the second valve disc 9 is spring-biased toward the back end of the valve rod 13 with the coil spring 14 installed between such second valve disc 9 and temperature sensor 1a. 
The coil spring 10 as the biasing means is degenerated and provided between the first valve disc 8 and frame 11 so as to constantly bias the first valve disc 8 to a valve-closing position.
With this kind of thermostat device 5, the first valve disc 8 is positioned to open and close the cooling channel 3b, and the second valve disc 9 is positioned to open and close the bypass channel 3c, and the operation thereof is as follows.
In other words, as a result of the thermal expansion body 7c inside the temperature sensor 7a expanding due to the rise in the coolant temperature and thereby pressing the piston rod 7d, the operating member 5 will operate by resisting the biasing force of the coil spring 10. As a result, the first valve disc 8 will move to an open position to release the cooling channel 3b, and the second valve disc 9 will move the valve-closing position to close the bypass channel 3c. Further, as a result of the coolant temperature falling, the thermal expansion body 7c will contract, the suppress strength of the piston rod 7d will weaken, the first valve disc 2 will move to the valve-closing position due to the biasing force of the coil spring 10 to close the water channel 3b, and the second valve disc 9 will move to the valve-opening position to release the bypass channel 3c. 
As described above, the thermostat device 5, as a temperature-sensing automatic valve, controls the temperature of the coolant to be sent to the engine water jacket 1 to be a suitable temperature by mixing and switching the heated coolant from the engine water jacket 1 and the cooled coolant from the radiator 2 inside the coolant circuit 3 of the engine.
With the thermostat device 5 having this kind of constitution, a proposal has been made in which a control plate for blocking the flow of the coolant is provided around the second valve disc 9 for opening and closing the bypass channel 3c, and, as a result of providing such a control plate, the coolant that passed through the bypass channel 3c can be smoothly guided to the temperature sensor 7a in the operating member 7, a mixing effect of the fluid is yielded in the vicinity of the operating member 7 thereby, and, by seeking the uniform temperature distribution in the vicinity of the operating member 7, the accurate control of the flow and amount of the coolant as well as superior responsiveness can be obtained (e.g., refer to Patent Document 1).
Patent Document 1:
Japanese Patent Kokai Publication No. H6-39190
With the thermostat device 5 employing a conventional structure, the bypass valve structure of the second valve disc 9 for opening and closing the bypass channel 3c is complex, and there are disadvantageous in that the number of components is numerous, assemblability is inferior, management of components is difficult, and there are problems in securing a reliable operation.
In other words, with the foregoing thermostat device 5 employing a convention structure, upon providing a first valve disc 8 to the guide unit 7b on one end of the operating member 7 as the thermoelement, the first valve disc 8 is press fitted and provided around the guide unit 7b, and it is also necessary to provide a stopper ring (i.e., C ring or E ring) to prevent such press fitting from disengaging. Thus, there were problems in that the number of components is numerous and the assemblability being inferior. Further, with this kind of structure, precision of the respective components for press fitting is required, and problems would also arise in terms of workability.
Moreover, with the foregoing conventional structure, since it is necessary to provide a valve rod 13, a second valve disc 9 to be provided with a stopper 13a to the tip of this valve rod 13, a coil spring 14 for biasing the second valve disc 9 and so on to the lower end of the operating member 7 as the thermoelement, the number of components is numerous, and the assembly was troublesome.
In addition, since the bypass valve as the second valve disc 9 is of a structure which opens and closes the opening end of the bypass channel 3c inside the housing, it is necessary to appropriately set the length of the valve rod 13 and the shape of the second valve disc 9 depending on the size or shape of the housing, and this will complicate the management of components and cause increased costs.
Further, with Patent Document 1 described above, since the precision of the temperature sensor 7a is secured by mixing the coolant, it is necessary to provide a control plate, and there is a problem in that the number of components will increase even more.